Measuring Ozone Deposition to the Ocean Surface and Assessing its Biogeochemical Controls

Dry deposition of ozone to the sea surface represents a significant portion of global tropospheric ozone loss. It introduces considerable uncertainty in global models due to limited understanding of the reactivity of iodide and organic material in the sea surface towards ozone. This is particularly true of organic material due to its variable composition. This thesis details ozone flux and associated measurements at and around the Penlee Point Atmospheric Observatory (PPAO) on the UK south coast from 2018 until 2021 where coastal ozone flux was calculated via eddy covariance. Monthly median deposition velocity was 0.007 – 0.033 cm s−1 across all fieldwork, similar to the values reported from ship-based measurements: 0.009 – 0.034 cm s−1. Iodide and dissolved organic carbon (DOC) concentrations in the water within the flux footprint were ~50 to ~100 nmol dm−3 and 1.3 – 2.2 mg dm−3. While iodide increased to a peak in July (coinciding with phytoplankton blooms), DOC conversely peaked in November. These measurements were used with a 1-layer and a 2-layer model to compare deposition observations to predictions. The 1-layer model in the absence of DOC reactivity typically gave values closest to observations and showed a similarly strong variation with friction velocity. Inclusion of the DOC-ozone reaction with a rate constant of 3.7 × 10−6 dm3 mol−1 s−1 caused both models to overestimate, but also mimic some variation between months suggesting its contribution was overestimated, but still important. Liquid chromatography-mass spectrometry was used to identify compounds in the water near the PPAO. Double bond equivalence decreased following exposure to 500 ppbv ozone, while dicarboxylic acid concentrations increased, potentially due to unsaturated fatty acid ozonolysis. Several dicarboxylic acid concentrations in PPAO samples fell from November – April, similarly to DOC concentrations. The potential for some introduction of dicarboxylic acids as contaminants from the sampling method remains a possibility.